Optical data storage is generally limited by the optical resolution of the read/write-system. Straightforward methods of increasing the optical resolution include using a shorter wavelength and a larger numerical aperture NA, at the costs of lens complexity. Further approaches are narrowing the allowable tilt margins for the optical storage media or reducing the wavelength of the scanning laser into the blue or near-UV range. A different approach for reducing the focus spot size in an optical data storage system is using near-field optics with a high numerical aperture (NA>1). This high numerical aperture is generally achieved by help of a solid immersion lens (SIL). While conventional systems like CD, DVD or BD operate in the optical far-field regime, which is described by classical optics, the aforementioned new systems work in the optical near-field regime, which is described by near-field optics. For conventional systems the working distance, i.e. the air gap between the surface of the optical storage medium and the first optical surface of the read/write-head, usually the objective lens, is in the scale of 100 μm. In contrast, systems making use of near-field optics need a very small working distance or air gap, which is in the scale of 50 nm or less. The small air gap is necessary to ensure that evanescent waves may couple into optical storage medium.
Usually the cover layer thickness of a near-field optical recording medium is not perfectly homogeneous due to the limitations of the spin coating process. Especially from the inner radius to the outer radius the thickness deviation is higher than at constant radius. Therefore, the thickness deviation has to be compensated by adjusting an optical element, e.g. a telescope or liquid crystal element. To this end WO 2005/104109 discloses a near-field optical data storage system using an objective including a solid immersion lens. The system includes means for adjusting an optical element in order to compensate for variations of the thickness of the cover layer of the near-field optical recording medium.
It is especially required to re-adjust the optics if the pickup jumps from the inner area of the near-field optical recording medium to another area at a higher radius or vice versa. To enable this re-adjustment it is advantageous to detect spherical aberration introduced by changes of the substrate thickness and radial tilt. An advanced pickup with five beams, which is capable of detecting spherical aberration introduced by changes of the substrate thickness and radial tilt, has recently been proposed in R. Katayama et al.: “Substrate thickness error and radial tilt detection using a five-beam optical head”, Appl. Opt. Vol. 48 (2009), pp. 2014-2026.